1. Field of the Invention
The coating or encapsulation of discrete particulate material by a variety of processes is well known. In particular, the art is aware of many variations in processes for coating particulate material by a fluidized bed technique.
The present invention finds utility for applying either a complete single coating or a double coating of a material to a particulate substance.
In the case of a single coating, encapsulation methods known heretofore do not always insure that the particles will have no adverse side effects during use.
The double coating process of the present invention is especially applicable to the coating of particles of highly reactive substances. Most particularly the process finds utility in the detergent bleach field wherein particulate fabric bleaching agents, such as potassium dichloroisocyanurate, sodium dichloroisocyanurate, and the hydrates thereof are employed in home laundering operations as a dry bleach product to be added separately to a washing machine or for use in admixture with particles of a detergent composition to form a commercially acceptable detergent -- dry bleach combination. Because of their highly reactive nature the particles must not only be thoroughly and imperviously coated to avoid contact of the bleach particles with the detergent particles, but the coated particles must not attack textile materials or the dyes thereon under washing conditions. Although prior-art processes may provide thorough and uniform coatings impervious to potassium iodide solution, the coated particles of chlorine-releasing compounds suffer the defect that they attack the dye substances at the points of contact with the fabric and have been known to attack the fabric and make pinholes therein in a washing process wherein the coated bleach particles are placed directly on dry clothes.
2. The Prior Art
Art that may be considered in connection with the instant invention is represented by the patents listed below.
U.S. Pat. No. 1,950,956 to Wilhelm
Discloses the coating of chloramine-T with stearic acid.
U.S. Pat. No. 1,480,561 to McNeil
Discloses a process wherein granules are uniformly coated by dropping granules through a spray of a solution of a coating substance and spraying the solution directly upon the granules while they are being agitated in a tumbling mechanism.
U.S. Pat. No. 2,561,392 to Marshall
Relates to process and apparatus for coating moving particles by spraying a solution of a coating material thereon. Provision is made to recirculate the particles to provide a heavier coating. There is also disclosed a means for removing the solvent from the coating material.
U.S. Pat. No. 2,579,944 to Marshall
Discloses a tandem coating system wherein particles in a turbulent state are coated by spraying with a solution of a coating substance in a first chamber and sequentially coated by spraying with a solution of a second substance in a second chamber. The patentee states in column 6, lines 18-22 that it is desirable to have the sprayers adjustable vertically so as to coat the particles properly when the surface of the bed is changed.
U.S. Pat. No. 2,594,469 to Mahoney
Relates to a process wherein particles, such as spray-dried detergent particles fall by gravity within a chamber, and the falling particles are coated by spraying with a solution of the coating substance.
U.S. Pat. No. 2,768,095 to Tadema et al.
Provides a method for uniformly distributing liquids in finely divided solids without substantial loss of materials. The liquid may be a solution of one or more materials in a suitable solvent, and is injected within a fluidized bed of the finely divided solids either counter to, or parallel to, the gas stream used for fluidization.
U.S. Pat. No. 3,650,961 to Hudson
Discloses a method for encapsulating chlorocyanurates with hydratable inorganic salts. A fluidized bed of the inorganic salt is formed, on which is sprayed droplets of a slurry of a chlorocyanurate, or other detergent adjunct. By controlling the droplet size, the detergent is made to form the core material disposed predominantly in the center of the particle, while the inorganic salt, partially hydrated, surrounds the core, and the particle size can be controlled to about 10-100 mesh.
U.S. Pat. No. 3,671,296 to Funakoshi et al.
Particles are coated by treatment with a liquid spray while the particles are circulating in a cyclic pattern. Circulation is accomplished by moving the particles to the outer edge of a rotating circular, horizontal dish, the force of this movement carrying the particles upward along the internal vertical surface of a cylindrical or inwardly curved barrel, then leaving the barrel surface to move toward the center of the barrel, thence falling back to the revolving plate to repeat the circulating process.